The present invention relates to a method for testing a balancing circuit for a battery that has a plurality of battery cells, to a device for testing the balancing circuit, to a battery system, and to a vehicle.
An electric vehicle or a hybrid vehicle has a battery system (high-voltage battery) that comprises a plurality of series-connected and/or parallel-connected batteries or battery modules that each have at least two battery cells. The discharging procedure for a battery that has a plurality of battery cells is stopped as soon as that battery cell of the battery that is charged to the least extent has reached its minimum voltage, as this battery cell would suffer damage and be destroyed if discharging were to continue. Likewise, the charging procedure for a battery has to be interrupted as soon as a battery cell of the battery has reached its maximum voltage. Continuing to charge this battery cell would lead to overcharging and thus to destruction. Following an interruption of the charging procedure or of the discharging procedure, however, the other battery cells connected in series inside the battery are likewise no longer charged or discharged, even though they are possibly not yet fully charged or discharged. For this reason, the state of charge of the entire battery is restricted by the battery cell having the highest or lowest battery cell voltage. The overall capacity of the battery may therefore be greatly impaired.
Since the charging procedure for a battery is restricted by the “fullest” battery cell and the discharging procedure for the battery is restricted by the “emptiest” battery cell, a battery system normally has a balancing circuit that serves to reduce the voltage difference between the “fullest” battery cell and the “emptiest” battery cell. To test whether the balancing circuit is working correctly, the current dissipated via resistors of the balancing circuit, according to the prior art, is measured by way of current sensors. If, following activation of the balancing circuit, no current flows through the resistors, the balancing circuit should be regarded as defective. As a current sensor is required for each individual battery cell in order to monitor the resistors, the costs of the battery system are increased due to the additional costs of the current sensors.
The object of the present invention is to provide a simple method for testing a balancing circuit, which method is able to be implemented at low cost. A further object of the invention is to provide a corresponding device for testing a balancing circuit, a battery system having such a device and a vehicle having such a battery system.
This and other objects are achieved by the method, device, system and vehicle in accordance with embodiments of the invention.
The method according to the invention for testing a balancing circuit for a battery that has a plurality of battery cells comprises the following steps:    a) detecting first voltage states of the battery cells by way of a control unit,    b) activating the balancing circuit for the purpose of achieving voltage equalization of at least two of the battery cells,    c) detecting second voltage states of the battery cells by way of the control unit, and    d) determining a functional capability of the balancing circuit on the basis of the first voltage states and the second voltage states of the battery cells by way of the control unit.
The control unit detects and compares the voltage states of the battery cells prior to and following a balancing procedure. If the balancing circuit is working correctly, the voltage difference between the “fullest” battery cell and the “emptiest” battery cell is reduced following the balancing procedure. If the voltage difference has not reduced or not reduced sufficiently following the balancing procedure, the balancing circuit should therefore be regarded as defective.
According to one development of the invention, step a) includes the following steps: a1) detecting a first group of voltages of the battery cells by way of the control unit; and a2) calculating a first voltage difference between a highest voltage and a lowest voltage of the first group of voltages by way of the control unit.
According to one development of the invention, step c) includes the following steps: c1) detecting a second group of voltages of the battery cells; and c2) calculating a second voltage difference between a highest voltage and a lowest voltage of the second group of voltages by way of the control unit.
The voltages of the battery cells are preferably able to be measured by way of a voltmeter, without using additional current sensors.
According to one development of the invention, step d) includes the following steps: d1) calculating a difference value between the first voltage difference and the second voltage difference and comparing the difference value with a prescribed voltage value by way of the control unit; and d2) determining, by way of the control unit, that the balancing circuit is not working if the difference value is smaller than a prescribed voltage value.
The functional capability of the balancing circuit is determined by virtue of the fact that the balancing circuit should be regarded as defective if the voltage difference, following the balancing procedure, has not reduced or has not reduced sufficiently, that is to say for example has reduced by at least a prescribed voltage minimum value or by a prescribed percentage of the original voltage difference.
According to one development of the invention, the method comprises a further step: determining, by way of the control unit, that the balancing circuit is working if the difference value is greater than or equal to the prescribed voltage value.
According to one development of the invention, step c1) is performed a specific time after step a1).
According to one development of the invention, the specific time is determined depending on a duration of the voltage equalization; or the specific time is a prescribed value. By way of example, step c1) may be performed depending on the duration of the voltage equalization, e.g. 1-10 hours, after step a1). As an alternative, the specific time may be prescribed by the system, e.g. as 2 hours.
According to one development of the invention, the prescribed voltage value is between 0 mV and 10 mV.
The present invention furthermore provides a device for testing a balancing circuit for a battery that has a plurality of battery cells. The device comprises a control unit. The device detects first voltage states of the battery cells and activates the balancing circuit for the purpose of achieving voltage equalization of at least two of the battery cells, and detects second voltage states of the battery cells and determines a functional capability of the balancing circuit on the basis of the first voltage states and the second voltage states of the battery cells.
According to one development of the invention, the device has at least one voltmeter; wherein the at least one voltmeter detects a first group of voltages of the battery cells; and wherein the control unit calculates a first voltage difference between a highest voltage and a lowest voltage of the first group of voltages.
According to one development of the invention, the at least one voltmeter detects a second group of voltages of the battery cells; wherein the control unit calculates a second voltage difference between a highest voltage and a lowest voltage of the second group of voltages.
According to one development of the invention, the control unit calculates a difference value between the first voltage difference and the second voltage difference and compares the difference value with a prescribed voltage value, wherein the control unit determines that the balancing circuit is not working if the difference value is smaller than the prescribed voltage value.
According to one development of the invention, the prescribed voltage value D4 is between 0 mV and 10 mV.
The present invention furthermore provides a battery system. The battery system comprises at least one battery that has a plurality of battery cells, a balancing circuit whose purpose is to achieve voltage equalization of at least two of the battery cells, and an abovementioned device.
The present invention furthermore provides an electric vehicle or hybrid vehicle having the abovementioned battery system.
As the voltages of the battery cells are already measured or monitored by the battery system for other reasons, it is advantageous that no additional component or sensor is required to implement the invention. Furthermore, the function, according to the invention, of the control unit is able to be implemented in an existing controller or an existing microcontroller. The invention thus provides a solution for testing a balancing circuit without increasing the costs of the battery system. The device according to the invention moreover does not require any extra installation space for positioning the control unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.